You haven't given much information. If you UPGRADED to Windows XP: Have you
tried booting into Safe Mode (Press the F8 key on boot BEFORE the Windows
logo). If you can boot into Safe Mode, then there's a probability that you
didn't run the Check System Compatibility wizard on the Windows XP Upgrade
CD, which would have listed what on your system isn't Windows XP compatible.
Often, people forget to uninstall their antivirus program (which usually
isn't compatible with Windows XP). Sometimes the fix is as simple as
downloading and installing the latest Windows XP compatible driver for your
Display Adapter, which can be found at the maker's web site. But, if none
of the above is applicable, and the system HAD been operating properly, and
now you have this problem: Boot into Safe Mode, take the option to go
directly to System Restore, and opt to restore your computer to an earlier
time, click next, click a date on the calendar that's in bold font (the
dates in bold font are the one's that have System Restore Points) and select
a date just prior to the problem, then in the right pane click to highlight
a Restore Point for that date, and finish the wizard. If that doesn't fix
the problem, it could be virus or defective hardware related. To determine
that, try doing a Repair Install, which keeps data and programs and repairs
Windows XP. Go to:
http://www.webtree.ca/windowsxp/repair_xp.htm
When the page opens, scroll down and click on: How To Run A Repair
Install. There is a step by step walk through including images. Your
software, settings, data files, desktop, etc. are retained.
Windows XP Repair Installs have save me a few times...